Commercial applications of white light emitting devices are manifold. White light emitting devices are not only used for lighting, but also in displays and as backlit. The requirements for modern white light emitting devices demand the development of new technologies. Displays in smartphones, tablet computers, TVs, and modern thin lighting devices, for instance, require thin and potentially flexible light sources with excellent performance characteristics. Thin electroluminescent organic light emitting devices, thus, represent an attractive target in order to accomplish specific requirements of new technologies. Particularly, organic light emitting diodes (OLEDs) and organic light emitting electrochemical cells (OLEC, LEC or LEEC) are considered as being promising electroluminescent devices.
Organic light emitting diodes (OLEDs), in which organic semiconductors are used as functional materials are disclosed, for example, in U.S. Pat. Nos. 4,539,507, 5,151,629, EP 0676461 and WO 98/27136. The organic light emitting materials being employed for this purpose are increasingly often organometallic complexes which exhibit phosphorescence instead of fluorescence (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). For quantum-mechanical reasons, an up to four-fold increase in energy and power efficiency is possible using organometallic compounds as phosphorescent emitters. In accordance with the prior art, the triplet emitters in phosphorescent OLEDs are, in particular, iridium and platinum complexes, which are typically cyclometallated. The emitters currently available emit light in the range from blue to red.
The scientific and industrial interest in OLEDs is particularly high due to their potential commercial applications for lighting and display technologies. OLEDs exhibit several advantages over existing and established technologies such as low energy consumption, low thickness and high flexibility of the devices. Furthermore, OLEDs represent a uniform irradiation source as compared to the spotlight of other light sources such as inorganic LEDs.
The possibilities to achieve white light emission in OLEDs (WOLEDs) are manifold. Single-color white OLEDs comprise one emitter with a broad emission spectrum, but achieving the desired color coordinates might be difficult with this basic approach. Therefore, multi-color approaches have been developed. The multi-color approach can be realized in different ways. Firstly, the emissive layer (EML) of an OLED can be multiple doped. Secondly, different emissive layers in a single OLED can be used (sublayer-approach). Thirdly, red, green and blue light emitting subpixel can be used to white emission. Moreover, different OLEDs can be stacked in order to get WOLDs (tandem OLED). K. Meerholz et al. provide an overview on white OLEDs in Adv. Mater. 2011, 23, 233-248.
However, OLEDs also have some disadvantages. The preparation of OLEDs is often quite complex and costly, as OLEDs exhibit a multi-layered structure comprising a number of different functional layers such as substrate, anode, hole injection layer (HIL), hole transport layer (HTL), emissive layer (EML), electron transport layer (ETL), electron injection layer (EIL), cathode and potentially further layers.
Depending on the specific technical application a suitable alternative to an OLED is an organic light emitting electrochemical cell (OLEC, LEEC or LEC). The preparation of OLECs—particularly if curves or three-dimensional surfaces occur—is less complex as compared to the preparation of OLEDs. This is due to the fact that the requirements relating to homogeneity of the layer is less stringent. Thus, the production costs in particular for mass production are much lower as compared to the ones of OLEDs.
Furthermore, OLECs do not rely on air-sensitive charge-injection layers or metals such as Ba or Cs for electron injection, which further simplifies their preparation and makes them more cost efficient, as compared to OLEDs. This is due to the less stringent requirements for encapsulation of OLECs.
The underlying technology of OLECs differs from the one of OLEDs or LEDs. Both OLEDs and LEDs are diodes with forward bias and reverse bias. In contrast to OLECs the I-V (current-voltage) curves of both OLEDs and LEDs are asymmetric. They represent semiconductor technologies whereas an OLEC is basically an electrochemical or more precisely an electrolytic cell. Charge transport in OLEDs occurs via the movement of holes and electrons from molecule to molecule (hopping) until holes and electrons form so called excitons, i.e. electron-hole-pairs. Light is emitted when electrons and holes recombine. In OLECs, upon applying a voltage, the electroactive compound, being a conjugated polymer or a small molecule, is oxidized (p-type doped) at the anode and reduced (n-type doped) at the cathode.
The p- and n-type doping regions grow in size until they meet to form a so called p-n junction. Further, an exciton is formed on the organic emissive compounds in the p-n junction. The radiative decay of the exciton leads to the emission of light. The original work and the principle of OLECs have been published by Qibing Pei et al. in Science, 1995, 269, 1086-1088. OLECs show symmetric I-V curves, have low driving voltages, and there is no need for reactive metals as cathode.
But the time needed for forming p-n junction is long. Therefore, the turn-on is not instantaneous. In addition, lifetime of OLECs is often very short as compared to highly efficient OLEDs. Thus, up to date OLECs aren't suitable for, e.g., display applications, but many other applications are possible such as light emitting label for, e.g. packaging.
One way to achieve white light emission with OLEDs is to use a conjugated polymer as emissive material in polymer light emitting diodes (PLEDs) as disclosed in EP 1670844 B1. The white copolymer as disclosed in EP 1670844 B1 comprises blue, red and preferably also green emitters. In order to get a broad emission in OLEDs, one has to tailor the composition of the light emitting components of the polymer so that energy transfer from the blue light emitting component to the green one and further to the red one is only partially. As energy transfer in conjugated polymers is believed to occur via Förster energy transfer, the composition of green emitter and especially that of red emitter has to be very low, usually less than 0.05 mol % related to the total polymer, or even less. This results in several problems such as 1) the color is very sensitive to the concentration of red emitter. Small variations regarding the concentration can cause significant changes of the color and 2) a trace of the red emitter may also cause difficulties regarding reliability of the preparation of the polymer and 3) traces of red emitters might cause color shifts depending on the applied voltage. It is well accepted on the prior art, that high efficient light emission in organic electroluminescent devices can be attained by the use of phosphorescent emitters. For this reason the polymers as disclosed in EP 1670844 B1 comprise also phosphorescent metal complexes as repeating units. However, due to so called “long-range resonantly enhanced triplet formation” one skilled person will expect that a mixture or blend of a phosphorescent metal complex and a conjugated polymer will effectively quench the emission of the polymer. White emission of such systems is either not possible or of insufficient quality.